1. Field of the Invention
The present invention generally relates to a digital level display device. It particularly relates to a digital level display device effectively used in a compact disk (CD) player, a digital audio tape recorder (DAT), or the like.
2. Background of the Invention
In a PCM recording/reproducing apparatus such as a CD player, a DAT, or the like, an analog audio signal is digitized so as to be recorded on a recording medium. During reproducing, a reproduced digital signal is converted into an analog signal which is sent out as an output. FIG. 4 is a block diagram showing an example of the conventional digital level display device used, for example, for monitoring a level of a digital signal in such a PCM recording/reproducing apparatus as described above.
An absolute value signal of n bits representing an absolute value of a digital signal is applied to terminals 1, 2, 3, . . ., n in such a manner that the respective n bits are connected to the n terminals with a one-to-one correspondence. The most significant bit (MSB) and the least significant bit (LSB) of the absolute value signal are connected to the terminals 1 and n respectively. Assuming now that the bit applied, for example, to the terminal 2 is the most significant bit of all the non-zero bits (i.e., of logic "1" or high level), the output of an OR circuit 24 corresponding to the terminal 2 becomes logic "1". As a result, the output of an inverter 35 becomes logic "0" (low level), so that a current from a predetermined power source (+V) is allowed to flow through a resistor 55 into a light emitting diode 45 to make it light.
The output of the OR circuit 24 is applied to a lower-order OR circuit 25, and the output of the OR circuit 25 is applied to a lower-order OR circuit 26. The operation described above is repeated so that all the light emitting diodes 46, 47, . . . , located lower in order than the light emitting diode 45 are made light.
The arrangement is made such that the n-th bit down of the data drives the light emitting diode for display of -6n dB. Accordingly, the level of a digital signal can be indicated in steps of 6 dB. The gradation by 6 dB is not precise since a factor of two variation in voltage (one bit place) is a variation of 6.03 dB.
Logarithmic decoders 11, 12, 13, . . . are provided for making the display have a resolution of 3 dB. For example, in the case where the bits of the data at the terminals 1 and 2 are logic "0" and "1" respectively, and when the level of the data is not smaller than 3 dB above the level represented by terminal 2, the decoder 13 produces an output of logic "1". As a result, an OR circuit 23 produces an output of logic "1" and an inverter 34 produces an output of logic "0". As a result, the light emitting diode 44 and the light emitting diodes lower in order than the diode 44 are made to light.
Thus, the level of the digital signal is indicated as 3 dB, with the level of 0 dB at full-scale.
As described above, in the conventional digital level display device, a conversion circuit for logarithmically converting the absolute value signal for display in dB is constituted by the decoders 11, 12, . . . . A conversion circuit for converting the logarithmically converted signal into a displaying signal suitable to be displayed is constituted by the OR circuits 21, 22, . . . . A driving circuit is constituted by the inverters 31, 32, . . . . Display means is constituted by the light emitting diodes 41, 42, . . . , the resistors 51, 52, . . . , and the like. Therefore, there has been such a disadvantage that the step or resolution of the display is fixed. Further, there has been such a disadvantage that the conventional device cannot be applied to the case where a level of data is required to be numerically displayed in dB, even though the device can perform bar display or dot display by use of the light emitting diodes 41, 42, . . . .